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What is Aperture? What is Q Value? And why are they important?

Modified on: Mon, Feb 2, 2015 at 9:42 PM

The length of string sensed by the pickup contributes strongly to its tonal quality of the pickup. This "window" through which the pickup "looks" at the string ripples is called the "aperture" of the pickup.

Narrow aperture pickups (single coil) sense the shuttling string ripples most accurately and yield more clearly defined sound. Wide aperture pickups ("humbuckers") sense the ripples at a different time at each pole tip. The combination of both signals can diminish or even cancel some of the upper harmonics while enhancing the lower harmonics of the note. This effect produces the mellower tonality of humbuckers. When the separation between sensing coils is wide enough (like in a Strat in the intermediate switch positions) the cancellations between pulses from the same ripple can be clearly heard as “out-of-phase" sounds.

Pickups coils are real inductors with resonant peaks usually in the audio range. Their impedance rises from the DC resistance value to a maximum (at the resonance) of several hundred thousand ohms at several kilohertz and drops again at higher frequencies. The inductance of the coil must be measured at frequencies well below the resonance to avoid the effects of coil capacitance. The value of the winding capacitance labeled C in the figure below and the high frequency losses to magnets, core materials, shielding and other things near the coil (AC resistance) cannot be measured directly but can be implied from the impedance curve.

The frequency and Q of the resonant peak are the major features of the pickup response curve. The Q value of the pickup is the width of its response curve 3dB below the maximum divided into its resonant frequency. The higher the Q value the narrower and taller the resonant peak. The Strat features a treble peak at the coil resonance. In the 1C that peak has been damped out the response covers a wider frequency range.